1. Field of the Invention
This invention relates to a hydraulic tensioner with a relief valve for use to apply an appropriate tension to a timing belt, a timing chain or a like member of an engine for a vehicle.
2. Description of the Related Art
As a hydraulic tensioner for use to apply an appropriate tension to a timing belt, a timing chain or a like member of an engine for a vehicle, a hydraulic tensioner with a relief valve or with an orifice is frequently used in order to eliminate an excessive tension when it acts upon the timing belt, timing chain or like member.
A hydraulic tensioner with a relief valve is disclosed in Japanese Utility Model Laid-Open Publication No. HEI-3-12995. FIG. 5 is a schematic sectional view showing the hydraulic tensioner with a relief valve disclosed in the Japanese Publication just mentioned. Referring to FIG. 5, the hydraulic tensioner 60 with a relief valve shown includes a housing 61, a cylindrical plunger 62 mounted for sliding movement in the housing 61 and having an opening at one end thereof, and a spring 63 fitted in the inside of the plunger 62 and extending between the housing 61 and the plunger 62 for biasing the plunger 62 in a direction in which the plunger 62 projects from the housing 61 so that an outer end face 64 of the plunger 62 is resiliently pressed against a tensioner lever 65 to apply a tension to a chain 66 through the tensioner lever 65.
The housing 61 has an oil path 68 formed therein for supplying oil from a hydraulic pressure source such as an oil pump not shown into an oil chamber 67 in the inside of the plunger 62. Also a ball seat 69 has an oil path 70 formed therein for communicating the oil path 68 of the housing 61 and the oil chamber 67 with each other. A check ball 71 is biased toward the ball seat 69 by a coil spring 73 having one end contacting with a retainer 72. The ball seat 69, check ball 71, retainer 72 and coil spring 73 cooperatively form a check valve. The check valve permits oil from flowing from the oil path 70 into the oil chamber 67 but prevents the oil from flowing out from the oil chamber 67 to the oil path 70.
The plunger 62 has an oil path 74 which communicates with the oil chamber 67, and a sub oil chamber 76 which communicates with the oil chamber 67 through the oil path 74 and is formed from a plug 75 which closes up the other end of the plunger 62. A sleeve 77 is fitted for sliding movement in contact with an inner face of the sub oil chamber 76. Also a spring 78 is fitted in the sub oil chamber 76 between the sleeve 77 and the plug 75 for biasing the sleeve 77 so that the sleeve 77 may be pushed into the sub oil chamber 76. The plunger 62 further has an outlet port 79 formed radially therein. The outlet port 79 is communicated with the sub oil chamber 76 when the sleeve 77 is pushed back in a direction against the biasing force of the spring 78 by a hydraulic load applied in the oil chamber 67. Thus, the elements mentioned form a relief valve.
In the conventional hydraulic tensioner 60 with a relief valve, if slackening of the chain 66 occurs while the engine is operating, then the plunger 62 is allowed to be projected by the biasing force of the spring 63. Thereupon, the check ball 71 is simultaneously spaced away from the ball seat 69 to allow oil to be supplied into the oil chamber 67 through the oil path 68 of the housing 61 and the oil path 70 of the ball seat 69. Consequently, the tension of the chain 66 is maintained by the oil thus supplied to the oil chamber 67.
On the other hand, if an excessively high tension is applied to the chain 66 by an increase of the distance between the axes of sprocket wheels S1 and S2 or the like which arises from heat of the engine, then the plunger 62 is acted upon by a pressing load in the direction opposite to the biasing direction to the plunger 62 from the tensioner lever 65. Thereupon, the check valve acts to prevent the oil in the oil chamber 67 from flowing out. Consequently, the oil in the oil chamber 67, which is non-compressible fluid, prevents backward movement of the plunger 62. However, the sleeve 77 of the sub oil chamber 76 is acted upon by a hydraulic pressure generated in the oil chamber 67 so that it is slidably moved in the direction opposite to the biasing direction of the spring 78 to allow the oil from the oil chamber 67 into the sub oil chamber 76 to allow the plunger 62 to move back a little thereby to absorb the excessive tension acting upon the chain 66. If the pressing load to the plunger 62 further increases to retract the sleeve 77, then the sub oil chamber 76 and the outlet port 79 are communicated with each other thereby to open the check valve to allow the oil of the oil chamber 67 and the sub oil chamber 76 to be discharged through the outlet port 79 to allow the plunger 62 to move back a comparatively great amount thereby to eliminate the excessive tension of the chain 66. As the hydraulic pressure drops as a result of the discharge of the oil, the sleeve 77 is moved by the biasing force of the spring 78 until it closes up the outlet port 79 so that the oil chamber 67 may thereafter be kept in a fixed pressure condition. However, if the hydraulic pressure drops excessively, then the check valve operates to allow the oil to flow into the oil chamber 67 from the hydraulic source thereby to return the hydraulic pressure in the oil chamber 67 to its ordinary level.
Meanwhile, another conventional hydraulic tensioner with a orifice is disclosed in Japanese Utility Model Laid-Open Publication No. HEI-3-12043. FIG. 6 is a schematic sectional view showing the hydraulic tensioner with an orifice disclosed in the Japanese Publication just mentioned. Referring to FIG. 6, the hydraulic tensioner with an orifice is generally denoted at 80 and includes a housing 81, and a cylindrical plunger 82 mounted for sliding movement in the housing 81 and having an opening formed at one end thereof. An oil chamber 83 is defined by and between the housing 81 and the plunger 82.
The housing 81 has an oil path 84 formed therein for supplying oil from a hydraulic source such as an oil pump not shown to the oil chamber 83. A spring 85 is fitted in the plunger 82 and extends between the housing 81 and the plunger 82. The plunger 82 is acted upon by a projecting force provided by a hydraulic pressure from the hydraulic source and a biasing force of the spring 85, and applies a tension to a chain 86 under the projecting force mentioned.
A check valve 87 is interposed between the oil path 84 and the oil chamber 83 for permitting oil to flow from the oil path 84 into the oil chamber 83 but preventing the oil to flow backwardly from the oil chamber 83 to the oil path 84. Accordingly, oil having entered the oil chamber 83 cannot flow out from the oil chamber 83 thereby to prevent backward or retraction movement of the plunger 82. An orifice 88 is provided to permit such slow backward or retraction movement of the plunger 82 and exhibits its effect when the tension of the chain 86 becomes excessively high.
The orifice 88 is provided in the plunger 82. In particular, a plug 90 is mounted in a hole 89 perforated at an end of the plunger 82, and the orifice 88 is provided in the plug 90. A check valve 92 is formed at an outward opening 91 of the plug 90 for permitting oil from flowing out through the orifice 88 but preventing admission of air into the orifice 88 from the outside. The check valve 92 includes a check ball 93 positioned in the outward opening 91, a retainer 94, and a spring 95 for biasing the check ball 93 into the outward opening 91. An oil path 97 is formed in a tensioner lever 96, and oil discharged through the orifice 88 flows out from the oil path 97.
In the conventional hydraulic tensioner 80 with an orifice having such a construction as described above, oil having flowed into the oil chamber 83 cannot flow out rapidly from the oil chamber 83 due to an action of the check valve 87 between the oil path 84 and the oil chamber 83 and an action of the orifice 88. However, when a high hydraulic pressure applies continuously to the oil, the orifice 88 permits the oil to flow out slowly therethrough. Due to the actions, even if the tension to the chain 86 increases suddenly, it can be returned to an appropriate tension.
On the other hand, if the tension to the chain 86 drops, then the plunger 82 tends to project. If a projecting phenomenon of the plunger 82 occurs when sufficient pressure oil is not supplied into the oil chamber 83 because of a drop in functioning of the oil pump, choking up of the oil filer or the like immediately after the engine is started, then the oil chamber 83 is placed into a negative pressure condition. Accordingly, the oil tries to enter the oil chamber 83 through the check valve 92. However, since supply of oil is insufficient, the negative pressure is transmitted to the outward opening 91. However, since the check valve 92 is provided in the outward opening 91, the check valve 92 is closed and the admission path for air is closed up. Even when oil is not supplied sufficiently in this manner, air is prevented from entering the oil chamber 83.
In the conventional hydraulic tensioner 60 with a relief valve described with reference to FIG. 5, since the sleeve 77 which is one of components of the relief valve generally has a glass-like shape, it has a large area over which it is acted upon by a hydraulic pressure. Accordingly, the spring 78 for biasing the sleeve 77 in the direction in which the sleeve 77 is pushed in must have a great spring force. However, the space in which the spring 78 is accommodated is limited. Consequently, the hydraulic tensioner 60 with a relief valve is disadvantageous in that an excessively high stress acts upon the spring 78 in the limited space and is liable to break the spring 78.
The hydraulic tensioner 60 with a relief valve is disadvantageous also in that, since the sleeve 77 itself has a great mass, it is low in responsibility in a high frequency region because of the inertia of the sleeve 77 itself.
Meanwhile, in the conventional hydraulic tensioner 80 with an orifice described hereinabove with reference to FIG. 6, the check valve 92 is provided adjacent the orifice 88 and includes the check ball 93. Accordingly, the check valve 92 has a function of preventing admission of air from the outside when the inside of the oil chamber 83 is put into a negative pressure condition. However, since the check valve 92 includes the check ball 93, the hydraulic tensioner 80 with an orifice is disadvantageous in that, if the check ball 93 is moved even a little by an influence of vibrations of the engine, oil in the oil chamber 83 leaks through the check valve 92.
It is an object of the present invention to provide a hydraulic tensioner with a relief valve wherein a force to be applied to a valve body of the relief valve from pressure oil can be reduced to reduce the stress to be applied to a spring for biasing the valve body thereby to augment the degree of freedom in designing of the relief valve.
In order to attain the object described above, according to the present invention, there is provided a hydraulic tensioner with a relief valve, comprising a housing, a plunger fitted for sliding movement in the housing, a first spring for biasing the plunger in a projecting direction in which the plunger projects outwardly from the housing, a high pressure oil chamber formed between the housing and the plunger, a check valve interposed between the housing and the high pressure oil chamber for permitting oil to flow into the high pressure oil chamber from the outside of the housing but preventing the oil from flowing out from the high pressure oil chamber, and a relief valve provided at a projecting end portion of the plunger. The relief valve has a pressure path communicating with the high pressure oil chamber, a valve body fitted for sliding movement in the pressure path, a second spring for normally biasing the valve body toward the high pressure oil chamber, and an outlet port for communicating, when the valve body is moved back against the biasing force of the second spring, with the pressure path to allow the oil to flow to the outside therethrough. The valve body is formed as a unitary member including a slidably movable portion fitted for sliding movement in the pressure path and acted upon by a hydraulic pressure of the high pressure oil chamber, and a flange portion having a cross sectional area greater than that of the slidably movable portion for being acted upon by the biasing force of the second spring.
In the hydraulic tensioner with a relief valve, the valve body of the relief valve is formed as a unitary member including a slidably movable portion fitted for sliding movement in the pressure path and acted upon by a hydraulic pressure of the high pressure oil chamber, and a flange portion having a cross sectional area greater than that of the slidably movable portion for being acted upon by the biasing force of the second spring. Consequently, the slidably movable portion of the valve body can be formed with a comparatively small cross sectional area, and consequently, the end face of the slidably movable portion is acted upon by a comparatively low force from a hydraulic pressure generated in the high pressure oil chamber. Accordingly, also the biasing force of the second spring which matches with the force applied to the end face of the slidably movable portion can be set to a comparatively low level. As a result, a spring having a comparatively low biasing force can be used for the second spring, and setting of a relief point within a limited space and at a high pressure can be anticipated.
Further, since the valve body of the relief valve is formed in such a manner as described above, the valve body itself can be made lighter in weight than ever, and this eliminates such a trouble that the valve body is opened by an inertial force of the valve body itself by high frequency vibrations of the entire apparatus.
Due to the advantages described above, the degree of freedom in designing a relief valve is augmented.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.